Because arrhythmia are profoundly influenced by autonomic tone, isoproterenol and adenosine are essential tools in the cardiac electrophysiology laboratory. Frequently arrhythmia are induced only during isoproterenol washout. By studying changes in autonomic tone before, during and after infusion of isoproterenol, the investigators expect to determine if a unique phase of sympatho-vagal balance occurs during washout which could explain this window of inducibility. Because adenosine causes first antiadrenergic changes and then sympathomimetic changes, this study will delineate the time course of both and allow optimum utilization in the cardiac electrophysiology laboratory of the autonomic changes produced by adenosine. Progress report and summary of findings: Twenty patients were studied. All completed the protocol consisting of monitoring sympathetic nerve recordings, blood pressure and heart rate variability before, during and after infusion of isoproteranol and administration of adenosine.